Bedrock is Concrete's front end library. It is used to support Concrete itself, Concrete's backend (the "Dashboard" theme that's built-in to Concrete CMS), Concrete's shipping theme (Atomik) and third party themes that want to ensure that they remain compatible with all major Concrete features. It is a superset of Bootstrap 5.

https://documentation.concretecms.org/tutorials/submitting-user-interface-pull-requests-for-concrete-cms-version-9